Linear compressor, particularly refrigerant compressor

ABSTRACT

The invention concerns a linear compressor ( 1 ), particularly a refrigerant compressor, with a first component group comprising a stator ( 18, 20 ) of a linear motor ( 4 ) and a cylinder ( 8, 10 ), a second component group comprising a reciprocating piston ( 16 ) and an armature ( 22 ) of the linear motor ( 4 ) as well as a piston rod ( 28 ) connecting the armature ( 22 ) to the piston ( 16 ), an oil sump in a housing ( 2 ) and an oil pump ( 38 ), the second component group being movable in relation to the first component group. It is endeavoured to provide a simple design ensuring a lubrication of the linear compressor. For this purpose, the oil pump ( 38 ) has a pump housing ( 40 ) that is permanently connected to the piston rod ( 28 ), the pump housing ( 40 ) immersing with at least one suction opening ( 43; 43   a,    43   b ) into the oil sump ( 41 ), the oil pump ( 38 ) supplying oil to the inside of the piston rod ( 28 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

Applicant hereby claims foreign priority benefits under U.S.C. § 119 from German Patent Application No. 10 2005 038 784.5 filed on Aug. 17, 2005, the contents of which are incorporated by reference herein. This Application relates to German Patent Applications No. 10 2005 038 783.7 (Attorney Docket No. 6495-0168); No. 10 2005 038 785.3 (Attorney Docket No. 6495-0170); No. 10 2005 038 781.0 (Attorney Docket No. 6495-0172); No. 10 2005 038 780.2 (Attorney Docket No. 6495-0173), filed on the same date herewith.

FIELD OF THE INVENTION

The invention concerns a linear compressor, particularly a refrigerant compressor, with a first component group comprising a stator of a linear motor and a cylinder, a second component group comprising a reciprocating piston and an armature of the linear motor as well as a piston rod connecting the armature to the piston, an oil sump in a housing and an oil pump, the second component group being movable in relation to the first component group.

BACKGROUND OF THE INVENTION

With such a linear compressor, a corresponding electrical supply of the stator will make the armature reciprocate in the stator. The armature drives the piston in a likewise reciprocating movement. The stator is connected to the cylinder, so that the piston is moved in the cylinder, thus increasing and reducing a compression volume.

During operation such a compressor must currently be supplied with oil. The oil has two tasks. Firstly, it lubricates parts, which move in relation to each other. Secondly, it helps sealing a gap between the piston and the cylinder, so that the compression behaviour of the compressor is improved.

U.S. Pat. No. 6,089,352 shows a linear compressor as mentioned in the introduction. The oil pump is fixed on the stator. It has an oblong chamber, whose first end is connected to an oil sump and whose second end is connected to an oil reservoir surrounding the cylinder. During operation, the stator oscillates with the frequency, with which the piston moves in the cylinder. Through inertia forces, which act upon the oil in the chamber during this oscillating movement of the stator, the oil is transported to the oil reservoir. However, it is necessary that the chamber is filled from the beginning. An unfilled chamber cannot work.

U.S. Pat. No. 5,993,175 shows a further linear compressor, in which the oil pump is located in the stator. It has a pump chamber, in which is located a displacement element that is connected to the stator and the armature via springs. When the armature moves in relative to the stator, the displacement element starts oscillating and sucks oil via a suction pipe from the oil sump into the pump chamber. From here, the oil can reach the piston-cylinder-gap of the compressor via several openings. The displacement element displaces excessive oil from the pump chamber via an outlet opening.

US 2004/0052658 A1 shows a further linear compressor, in which the stator is connected to an oil pump, which has a pump body that is immersed in the oil sump. The pump body has an opening, which extends perpendicularly to the movement direction of the armature. During operation, the stator oscillates as a reaction to the movement of the armature. Through the opening in the pump body oil from the oil sump can enter, while the other end of the pump body is connected via a pipe to an oil passage formed in the stator, the oil passage ending in the gap between the piston and the cylinder. In this connection, the cylinder is formed by the inside of the stator and the piston is located in the stator.

Such oil pumps have a relatively low delivery rate.

JP 2000 154 778 A2 shows a linear compressor with an oil pump. The oil pump has a pump chamber immersing in the oil sump, the pump chamber being formed as a cylinder, in which a piston moves. The piston is connected to the armature. During a stroke of the armature in one direction, the piston displaces oil from the pump chamber into an oil reservoir, which again supplies the gap between the piston and the cylinder and a piston rod bearing with oil. During a return stroke of the armature, the oil pump piston sucks in oil through another opening. The path back from the oil reservoir is blocked via a non-return valve. Such a pump supplies an increased amount of lubricant. However, it is relatively expensive to manufacture and requires a certain space inside the compressor housing.

BRIEF SUMMARY OF THE INVENTION

The invention is based on the task of providing a simple design to ensure lubrication for a linear compressor.

With a linear compressor as mentioned in the introduction, this task is solved in that the oil pump has a pump housing that is permanently connected to the piston rod, the pump housing immersing with at least a suction opening into the oil sump, the oil pump supplying oil to the inside of the piston rod.

The design of such an oil pump is relatively simple and only requires little space. As the pump housing is permanently connected to the piston rod, they reciprocate synchronously. The piston rod has the same stroke as the piston, so that also the pump housing with its suction opening will be moved through the oil sump via a corresponding stroke. Thus, a sufficient amount of oil will reach into the pump housing, the oil being supplied from here to the inside of the piston rod. The piston rod is then used as auxiliary means for transporting the oil to the area of piston and cylinder.

It is preferred that the oil pump is located at the end of the piston rod facing away from the piston. This embodiment has several advantages. In a manner of speaking, the free end of the piston rod facing away from the piston projects from the stator, so that the pump housing is free to move. Design measures for preventing a collision between the pump housing and other parts of the linear compressor are not necessary. Secondly, the oil is transported through the linear motor, and is thus able to dissipate the heat occurring here. This will heat up the oil and reduce its viscosity, so that it gets highly liquid. This again reduces frictional losses between the piston and the cylinder.

Preferably, the piston rod has a channel, which is connected to a piston joint. In order to equalise alignment errors, it may be favourable not to locate the piston rigidly on the piston rod, but via a joint, for example, a ball joint. In order to keep the friction small here in spite of possibly occurring small movements, the oil from the oil pump is supplied directly into this joint.

Preferably, the piston rod is supplied with at least one pressure equalisation opening. The channel inside the piston rod will usually not be completely filled with oil. On the contrary, due to gravity the oil will only fill a partial area of the cross-section. Over the oil a gas volume then remains, which is connected to the inner chamber of the compressor housing via the pressure equalisation opening. Thus, the pressure equalisation opening permits a pressure equalisation, which is particularly advantageous, when leakage gas is pushed backwards from the compression chamber through the lubrication channels inside the piston. An involved pressure build-up could counteract the supply efficiency of the oil pump. This is reliably prevented by the pressure equalisation opening.

Preferably, the piston rod has a connecting element to the piston, whose inner diameter tapers. Thus, a steadily supplied amount will increase the pressure, so that the oil can leave towards the piston at a certain pressure.

It is also preferred that at least one inner diameter reduction is located inside the piston rod. This inner diameter reduction is then some kind of return flow prevention, the return flow prevention managing without movable parts. Nevertheless, within certain limits, it has the same effect as a non-return valve.

This is further improved in that a recess is located adjacent to the inner diameter reduction. A movement of the piston rod in the direction of the cylinder will make the oil dam up in this recess, as basically the oil film is inert and will not on its own follow the movement of the piston rod. When the piston rod is moved in the opposite direction, the inertia of the oil transported through the inner diameter reduction and into the recess will cause it to remain in the position, to which it has been transported, so that subsequent movements of the piston rod over a short period will transport the oil from the pump housing to the position, in which it will evolve its effect.

Preferably, the piston rod has a suction end, whose outer diameter reduces in a direction away from the piston. At least section-wise, the suction end can also have a slightly conical shape. This reduces the mass of the piston rod at this end. At the same time, fixing the piston rod on the side facing away from the piston will only require smaller surface. This is particularly advantageous, when this end is fixed in a resonance spring arrangement.

It is preferred that the suction end has a suction channel, which ends inside the piston rod and has a smaller inner diameter than the piston rod, the end of the suction channel being surrounded by a projection pointing in the direction of the piston. This causes that a relatively large oil volume is permanently available inside the piston rod. At the same time, however, the cross-section in the suction area is kept small, so that smaller pressures are required to transport the oil from the oil sump to the level of the piston rod.

Preferably, the pump housing has a pipe, whose one end is connected with the piston rod, immersing together with the suction opening in the oil sump, the normal to the surface of the suction opening having a component, which is parallel to the movement direction of the first component group. The normal to the surface can also be called the axis of the suction opening. When the normal to the surface or the axis is parallel to the movement direction of the first component group, that is, has a component being parallel to the movement direction of armature, piston rod and piston, the movement of the suction opening will cause oil to be pressed into the suction opening and then, via the pipe, into the inside of the piston rod.

It is preferred that the normal to the surface is parallel to the movement direction. In this case, the total cross-section of the suction opening is available for the entry of the oil in the movement direction of the piston rod.

In a preferred embodiment, it is ensured that the pump housing has two suction openings in the oil sump, the normals to the surfaces of the suction openings each having a component, which is parallel to the movement direction of the first component group, the components having opposite directions. In this case, a movement of the piston rod in each direction will give an oil supply, that is, oil will be supplied through the pump housing into the inside of the piston rod in connection with both a suction stroke and a pressure stroke of the piston.

It is preferred that a blocking element is located between the two suction openings, said blocking element preventing a straight flow of oil from one suction opening to the other. This keeps losses small.

It is preferred that the blocking element is movable. The movement can be initiated by the inertia of the blocking element or by the pressure of the available lubricating oil or by both in common. An additional energy supply or control of the blocking element is thus not required.

It is preferred that the blocking element exists in the form of a valve element, which is movable between a first valve seat that is allocated to one suction opening and a second valve seat that is allocated to the other suction opening. In this case, the blocking element always blocks the suction opening, through which oil is not presently pressed into the pump housing.

In an alternative embodiment it may be provided that each suction opening is closed by a spring element, which can be opened by the available oil pressure. Also in this case the passive suction opening is closed, so that oil cannot escape from the pump housing.

It is preferred that both spring elements are formed by a common spring ring. This simplifies the mounting.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is described on the basis of a preferred embodiment in connection with the drawings, showing:

FIG. 1 is a schematic longitudinal section through a linear compressor;

FIG. 2 is an enlarged view of a piston rod with piston and oil pump;

FIG. 3 is a modified embodiment of an oil pump;

FIG. 4 is a longitudinal section through the oil pump according to FIG. 3;

FIG. 5 is a third embodiment of an oil pump in a section V-V according to FIG. 6;

FIG. 6 is a section VI-VI according to FIG. 5;

FIG. 7 is a fourth embodiment of an oil pump in a perspective view; and

FIG. 8 is a sectional view for explaining an oil pump according to FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a linear compressor 1, which is located in a hermetically closed capsule 2.

The linear compressor 1 has a compression section 3, a drive section 4 and a resonance spring arrangement 5. The unit formed by the compression section 3, the drive section 4 and the resonance spring arrangement 5 is suspended in the capsule 2 via two plane annular springs 6, 7, each formed as a spiral with one winding. The annular springs 6, 7 are fixed on the drive section 4.

The compression section 3 has a cylinder 8, whose one front side is covered by a cylinder head 9. By means of a capsule 10, the cylinder 8 and the cylinder body 9 are assembled in the form of a cartridge. A suction muffler 11 and a pressure muffler 12 are fixed on the cylinder head 9. The suction muffler 11 is connected with a suction opening 13 and the pressure muffler is connected with a pressure opening 14 in the cylinder head.

The capsule 10 is inserted in an intermediary ring 15, which is connected with the drive section 4. During mounting, the capsule 10 and thus the cylinder 8 can be displaced within certain limits in the axial direction of the cylinder relative to the intermediary ring 15. When, as will be explained below, a predetermined position of the cylinder in relation to the drive section 4 has been reached, the capsule 10 is fixed in the intermediary ring 15, for example by welding, soldering or gluing.

In the cylinder 8 is located a piston 16, which borders a compression chamber 17 together with the cylinder 8 and the cylinder head 9. Before fixing in the capsule 10 in the intermediary ring 15, the piston is then expediently moved to its upper dead point (in relation to FIG. 1: to the right) and the cylinder 8 with the capsule 10 is displaced so that the compression chamber 17 assumes a minimum size.

The drive section 4 has a linear motor. The linear motor has an outer stator 18 with a recess 19 for a winding, not shown in detail, and an inner stator 20. Between the outer stator 18 and the inner stator 20 is an annular gap 21, in which an armature 22 is movable. The armature carries permanent magnets 23, which are connected with each other by means of two rings 24, 25. The rings 24, 25 can, for example, be made of plastic. The rings 24, 25 are connected to inner rings 26, 27 via arms, not shown in detail, which are guided through slots in the inner stator 20.

The inner rings 26, 27 are connected with a piston rod 28, which again is connected with the piston 16. FIG. 2 shows an enlargement of the piston rod with further details.

The outer stator 18 and the inner stator 20 are connected with each other via motor covers 29, 30, which are tied to each other by means of screw bolts 31. The screw bolts are guided in parallel to the movement direction of the piston rod 28. The piston rod 28 is guided through the motor covers 29, 30 in a touch-free manner.

The intermediary ring 15 is connected with the cylinder side motor cover 30, for example by welding, gluing or soldering.

The resonance spring arrangement 5, which is located at an end of the drive section 4 opposite the compression section 3, has a spring pack 32 comprising several plate springs 33. The spring pack 32 is connected to the piston rod 28 in a central area 34. An outer section 35 of the spring pack 32 is connected by means of bolts 36 to a stop housing 37, which forms a stop for the spring pack 32.

When the winding located in the recess 19 is provided with current, the armature 22 moves in one direction and takes along the piston rod 28 in this direction. When the direction of the current is reversed, the armature 22 with the piston rod 28 moves in the opposite direction and accordingly moves the piston 16 in the opposite direction. This will periodically increase and decrease the volume of the compression chamber 17. The resonance spring arrangement 5 is adapted to the frequency of the current, so that the movable part of the linear compressor 1, which is formed by the armature 22, the piston rod 28, the piston 16, the oil pump arrangement 38 and the movable part of the resonance spring arrangement 5, oscillates in resonance.

At the end projecting from the spring pack 32, the piston rod 28 is connected to an oil pump, which immerses in an oil sump 41, merely schematically shown in FIG. 2, which forms in the bottom part of the capsule 2.

The oil pump 38 has a pump housing, which is immersed in the oil sump 41. The pump housing 40 is connected to the piston rod 28 by means of a rigid pipe 42. This means that the pump housing 40 moves synchronously with the piston rod 28.

The pump housing 40 has a suction opening 43, whose normal to the surface is parallel to the movement direction 44 of the piston rod 28. In other words, the axis of the suction opening 43 is parallel to the movement direction 44 or the suction opening is perpendicular to the movement direction 44. When the piston rod 28 is moved to the left, the oil from the oil sump 41 is pressed through the suction opening 43 into the pipe 42, the movement of the piston rod 28 transporting it into the hollow inside 45 of the piston rod 28. The term “suction opening” is chosen for reasons of clarity here, as the inlet opening of a pump is usually called suction opening. In this case, however, the supply process of the oil pump 38 is less based on suction and more on pressure.

At the piston side end, the channel 45 is connected to a piston joint 46 in the form of a ball joint. The piston joint 46 has a ball 47, which is adopted in a ball socket 48. The contact face between ball 47 and ball socket 48 can be lubricated with oil supplied through the channel 45.

The connection between the piston rod 28 and the piston 16 occurs via a connecting element 49, which tapers conically in the direction towards the piston 16. Thus, the inner diameter of the channel 45 decreases. In the area of the piston 16 is provided a pressure balancing opening 50. The channel 45 inside the piston rod is usually not completely filled with oil, but only in the bottom part. Through the lubricating channels in the piston 16 leakage gas could also be pressed back from the compression chamber into the channel 45. To prevent this leakage gas from building up a pressure in the channel 45, which again would counteract the supply effect of the oil pump, the pressure balancing opening 50 is provided for generating a pressure balancing to the inside of the compressor housing.

The connecting element 49 is attached on the piston rod 28. It is fixed by frictional forces. If required, it can also be fixed on the piston rod by gluing, welding or soldering. On a whole, it is assumed that also the connecting element 49 is a part of the piston rod 28.

The inside of the piston rod has at least one diameter reduction 53. In the present embodiment, it is formed at the transition between the piston rod and the connecting element 49. Next to the diameter reduction is formed a recess 54. In the present case, the recess is realised in the form of an insert 55, which is inserted into the channel 45 and has an outlet cone 56 on its piston side end.

The piston rod 28 has a suction end 57, whose outer diameter reduces in a direction away from the piston 16. The suction end 57 can be screwed onto the piston rod 28 or be glued, welded or soldered onto the piston rod 28. The diameter reduction of the suction end 57 causes that less space is required for fixing the piston rod 28 in the spring pack 32.

The suction end 57 has a suction channel 58, which has a smaller inner diameter than the channel 45 of the piston rod 28. The end of the suction channel 58 into the channel 45 is surrounded by a projection 59, which points in the direction of the piston 16. Radially outside the projection a recess 60 forms.

As shown schematically, a return flow prevention device 61 can also be located in the channel 45, for example in the form of a saw tooth profile, whose piston side end has sides, which are perpendicular or inclined towards the piston 16, whereas the other sides have a smaller inclination. It is also possible to arrange further “throttling spots” in the channel 45, which have embodiments similar to those formed by the insert 55 or the projection 59.

The oil supply through the unit consisting of piston rod 28 an oil pump arrangement 38 as shown in FIG. 2 can be described as follows:

During operation, the armature 22 reciprocates together with the piston 16 and the oil pump 38. The frequency of this movement corresponds to the frequency of the a.c. supply to the linear motor.

When the piston rod 28 is moved to the left (in relation to the view in FIG. 2, oil from the oil sump 41 will be pressed into the suction channel 58 through the suction opening 43 and the pipe 42. As the stroke length of the suction opening 43 corresponds to the stroke length of the piston 16 in the cylinder 8, the supplied amount of oil is sufficient to reach the suction channel 58. The inertia of the oil causes that at least a share of the oil supplied to the suction channel 58 remains there. Repeated movement strokes of the piston rod 28 will thus eventually fill the suction channel 58, which runs over into the channel 45.

When the suction channel is filled and the oil runs into the channel 45, the recess 60, which surrounds the projection 59, will prevent it from completely flowing back into the suction channel 58. The inertia of the oil, which is pushed in the direction of the cylinder 8 by the projection 59, will prevent it from flowing back during a return movement of the piston rod 28. On the contrary, it will eventually get through the insert 55 into connecting element 49. From here, it cannot either completely flow back into the channel 45, as this is prevented by the outlet cone 56. The oil available in the connecting element 49 can thus only flow on into the ball joint 46. Additionally, the return flow of the oil can also be prevented or blocked by the return flow prevention device 61.

The FIGS. 3 and 4 show a modified embodiment of the oil pump arrangement 38. The same elements have the same reference numbers as in FIGS. 1 and 2.

In this case, the pump housing 40 has two suction openings 43 a, 43 b, which are located opposite each other in the movement direction 44. Between the two suction openings 43 a, 43 b is located a stop element in the form of a wall 62, which prevents a straight flow of oil from the suction opening 43 a to the suction opening 43 b or vice versa.

In principle, this oil pump arrangement 38 works exactly as explained in connection with FIG. 2. However, here an oil supply occurs with each movement direction. When the pump housing 40 is moved to the left, oil is pressed from the oil sump 41 through the suction opening 43 a into the pump housing 40. When the pump housing 40 is moved to the right, oil is pressed from the oil sump 41 through the suction opening 43 b into the pump housing 40. Here, the direction details refer to the view in FIG. 4.

The FIGS. 5 and 6 show a third embodiment of an oil pump housing 40. Also this oil pump housing 40 has two suction openings 43 a, 43 b located opposite each other. The pump housing 40 has an approximately circular inner cross-section. A spring plate 63 bent into cylinder shape and covering both suction openings 43 a, 43 b is inserted into this inner cross-section. The spring characteristic of this spring plate 63 is relatively soft, so that already small pressures will be sufficient to deform the spring plate 63 so much, that one of the two suction openings 43 a, 43 b is released.

When the pump housing is moved to the left (in relation to the view in FIG. 5), the oil available in the oil sump 41 will press an arm 63 a of the spring plate 63 into the inside of the pump housing 40, and the oil can then flow past the arm 63 a into the inside of the pump housing 40. However, it cannot escape through the oppositely located suction opening 43 b, as the incoming oil presses the other arm 63 b firmly against the inner wall of the pump housing 40 and closes the suction opening 43 b. The same applies for a movement of the pump housing 40 to the right. In this case, the arm 63 b is opened by the available oil and the arm 63 a is kept closed.

The FIGS. 7 and 8 show a fourth embodiment of a pump housing 40, which again has two suction openings 43 a, 43 b. The suction opening 43 a is allocated to a valve seat 64a and the suction opening 43 b is allocated to a valve seat 64 b. A valve element 65, here working as a blocking element, can move between the two valve seats 64 a, 64 b. It therefore comes to rest on the first valve seat 64 a or on the second valve seat 64 b.

The movement of the valve element 65 is supported by two factors. Firstly, the valve element 65 has a certain inertia, so that a movement of the pump housing 40 to the right (in relation to the view in FIG. 8) will bring it to the left valve seat 64 a. This movement is also supported by the oil flowing in through the suction opening 43 b. When, however, the pump housing 40 is moved to the left, the valve element 65 is moved to the right in the pump housing 40 and reaches the valve seat 64 b. Oil from the oil sump 41 can thus only flow in through a suction opening 43 a or 43 b. An escape through the other suction opening 43 b, 43 a is prevented.

While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention. 

1. A linear compressor, particularly a refrigerant compressor, with a first component group comprising a stator of a linear motor and a cylinder, a second component group comprising a reciprocating piston and an armature of the linear motor as well as a piston rod connecting the armature to the piston, an oil sump in a housing and an oil pump, the second component group being movable in relation to the first component group, wherein the oil pump has a pump housing that is permanently connected to the piston rod, the pump housing immersing with at least one suction opening into the oil sump, the oil pump supplying oil to the inside of the piston rod.
 2. The linear compressor according to claim 1, wherein the oil pump is located at the end of the piston rod facing away from the piston.
 3. The linear compressor according to claim 1, wherein the piston rod has a channel, which is connected to a piston joint.
 4. The linear compressor according to claim 1, wherein the piston rod is supplied with at least one pressure equalisation opening.
 5. The linear compressor according to claim 1, wherein the piston rod has a connecting element to the piston, whose inner diameter tapers.
 6. The linear compressor according to claim 1, wherein at least one inner diameter reduction is located inside the piston rod.
 7. The linear compressor according to claim 6, wherein a recess is located adjacent to the inner diameter reduction.
 8. The linear compressor according to claim 1, wherein the piston rod has a suction end, whose outer diameter reduces in a direction away from the piston.
 9. The linear compressor according to claim 8, wherein the suction end has a suction channel, which ends inside the piston rod and has a smaller inner diameter than the piston rod, the end of the suction channel being surrounded by a projection pointing in the direction of the piston.
 10. The linear compressor according to claim 1, wherein the pump housing has a pipe, whose one end is connected with the piston rod, immersing together with the suction opening in the oil sump, the normal to the surface of the suction opening having a component, which is parallel to the movement direction of the first component group.
 11. The linear compressor according to claim 10, wherein the normal to the surface is parallel to the movement direction.
 12. The linear compressor according to claim 10, wherein the pump housing has two suction openings in the oil sump, the normals to the surfaces of the suction openings each having a component, which is parallel to the movement direction of the first component group, the components having opposite directions.
 13. The linear compressor according to claim 12, wherein a blocking element is located between the two suction openings, said blocking element preventing a straight flow of oil from one suction opening to the other.
 14. The linear compressor according to claim 13, wherein the blocking element is movable.
 15. The linear compressor according to claim 14, wherein the blocking element exists in the form of a valve element, which is movable between a first valve seat that is allocated to one suction opening and a second valve seat that is allocated to the other suction opening.
 16. The linear compressor according to claim 13, wherein each suction opening is closed by a spring element, which can be opened by the available oil pressure.
 17. The linear compressor according to claim 16, wherein both spring elements are formed by a common spring ring. 